This invention relates to a video server which delivers a video program to subscriber terminals in a video-on-demand (abbreviated to VOD) system.
With the latest development of a digital transmission technique and a compression and expansion technique of a digital signal, a video service system has been researched and noticed which provides an interactive video service between a plurality of subscribers or subscriber terminals and a video server and which may be generally called a video-on-demand (VOD)system It has been predicted that such a VOD system would be widely used in the future in the world.
Herein, such a VOD system can be distinguished from a CATV system of a broadcast type in that each subscriber terminal is in one-to-one correspondence with each of the video programs in the VOD system and interactive control or service can be executed between the subscriber terminal and the video server.
Description has been made about a conventional VOD system in the paper which is published by Hatem Ghafir et al in Proceedings of IEEE GCOM '94, pages 886 to 890 and entitled "Multimedia servers-design and performance". Similar system has also been published in the U.S. Pat. No. 5,247,347 issued to Larry A. Litteral et al. Such conventional VOD systems have a video server including a plurality of video storage units and might provide an interactive video service from the video server to each of subscriber terminals in principle.
However, the conventional VOD systems have a lot of problems to be solved when such systems are put into practical use. This is because adequate consideration has not been given to various kinds of request commands and the like which are sent from the subscriber terminals and about functions of the video server required to process the various commands.
More specifically, when a subscriber terminal requests transmission of a video program to a video server, a nontransmission time or an idle time during which no video program is transmitted to the subscriber terminal inevitably takes place in the conventional systems. For example, such a nontransmission time or an idle time occurs while the subscriber terminal requests a certain video program and waits for transmission of the video program requested by the subscriber terminal. In this event, the idle time is mainly determined by a time which is spent until the video storage unit is put into a reproducible state of the video program stored in the video storage unit. Specifically, when the video program is reproduced from a video cassette recorder prepared as the video storage unit in the video server, the idle time depends on a response time for searching for a leading portion of the video program in question in the video cassette recorder. Practically, the response time is varied in the video cassette recorder from less than one second to several tens of seconds depending on concentration of requests to the video cassette recorder.
At any rate, such an idle time occurs more or in the video cassette recorder. In the conventional VOD systems, the idle time is wasted in vain without providing any service and is therefore inconvenient for both a service provider and subscribers.
Herein, it is to be noted that various kinds of request control commands, such as a "PAUSE" command, a "FAST-FORWARD" command, a "FAST-REWIND" command, are sent from subscriber terminals to the video server in the conventional VOD systems with a video program received by the subscriber terminals, to transiently stop, to quickly forward, to quickly rewind the video program, and the like. Such "PAUSE" command, "FAST-FORWARD" command, "FAST-REWIND" command may be collectively referred to as specific reproduction control commands.
Supplied with the specific reproduction control commands from the subscriber terminals, the video server executes each of the specific reproduction control commands. During the execution of the specific reproduction control commands, the subscriber terminals can know the specific reproduction control commands that are being executed by a message displayed but can not receive any video service. This shows that a nontransmission time or an idle time occurs during execution of the specific reproduction control commands.
Furthermore, it should be noted that an advertisement or commercial video program may be prepared by a program sponsor or supplier who pays commercial fees to a service provider. On the other hand, each of the subscribers can send the reproduction control commands to the video server, as mentioned above. Under the circumstances, it is possible for each subscriber to skip or neglect such an advertisement or commercial video program by the use of such a reproduction control command, for example, the "FAST-FORWARD" or the "FAST-REWIND" commands As a result, the commercial video program is not watched by the subscribers. This results in a reduction of efficiency of the advertisement or commercial program given by the program sponsor and finally brings about a decrease of advertising revenue for the service provider.